It is well-known that the breakdown voltage, the on-resistance and the reliability are the most important issues of power devices.
During turning-on and turning-off of lateral high-voltage power devices, there is a transient stage with high voltage and large current. When a unipolar lateral power device operating in this stage, the carriers introduced by the large-current density produce a significant deviation of distribution of electric flux density of the lateral device from that case of without carriers, making the electric field of a local place be enhanced and therefore the impact ionization coefficients increased, resulting a local avalanche breakdown. The SOA (Safe Operating Area) of the device is then reduced by such a local avalanche breakdown, and the reliability of the device is also decreased.
To diminish such an effect, a conventional measure is using a small current density in the turn-on stage. However, such a method is at an expense of the performance of the device. Also, a perfect elimination of the deviation of electric flux density distribution of surface is impossible.